Effect of Cu doping in the structural, electrical, optical, and optoelectronic properties of Sol-Gel ZnO thin films

The effect of Cu doping (0.08-4%) on the structural, electrical, optical, and optoelectronic properties of sol-gel ZnO thin films deposited on glass substrates have been investigated. X-ray diffraction studies show that the doped films are single-phase wurtzite structure with random orientations. A decreasing trend in the c -axis parameter up to 0.5% is observed followed by an increasing trend above that. Field-emission scanning electron microscopy shows a grain texture. The energy dispersive spectroscopy as well as the film color confirms the Cu doping. Optical transmittance measurements show 80% transparency in the visible region for the films up to 0.3% Cu content. The doped films become very resistive as indicated by the four probe electrical resistivity and current-voltage measurements. The photoluminescence spectra reveal with Cu doping, a green emission at 515 nm appeared and the UV peak intensity at 376 nm is drastically lowered. However, no green emission is observed as the doping is increased beyond 0.5%. The changes in the structural, electrical, and optical properties are correlated to interstitials defects. The photoconductivity studies show that the UV sensitivity of the films is enhanced up to 0.1%, which indicates that the films can be used as UV sensors.